Clinical and morphologic features are described in a man who developed a pseudoaneurysm arising from the ventricular septum after transaortic septotomy for hypertrophic subaortic stenosis. Pseudoaneurysm of the left ventricle or ventricular septum has not been described previously in a patient with hypertrophic cardiomyopathy in whom only the transaortic approach was utilized for septotomy or septectomy or both. In addition, no previous reports have been described pseudoaneurysm arising from the ventricular septum.